A classifier of this kind, as already known from German OS No. 1,607,631, is used for classifying, i.e. separating a mixture or quantity of granular material, of different sizes and/or different shapes and/or different specific weight, into coarse material or coarse grains (of a grain-size above a certain diameter) and fine material (of a grain-size below that limit). The material to be classified is charged from above into a classifying area (an annular space between a vane-ring and a sifting rotor); at the same time, sifting air flows from the outside, through the vane-ring, into the classifying area, thus following a spiral path and carrying the particles along accordingly. In the classifying area, the particles are subjected to radially directed resistance and centrifugal forces acting in opposite directions. For a specific radial and axial air-velocity, there exists a limiting grain size at which the two forces are equal.
The coarser particles (coarser than the limiting size) thus arrive sooner or later at the coarse-material outlet at the bottom of the classifying area. The fine material, on the other hand, is carried along with the sifting air which flows inwardly through the passages in the sifting wheel and is drawn off through a fine-material/sifting-air outlet adjoining axially the interior of the sifting wheel. The fine material is finally separated from, or precipitated out of, the sifting air which can then be returned to the sifting-air inlet, i.e. it can be circulated. Classifiers of this kind are used mainly for recovering fine material of the smallest possible grain-size and in a narrow grain-size distribution, the upper limit sought being between 2 and 4 .mu.m This very fine granular material is frequently used as a filler, for synthetic material, automobile tires, colours, paints or coating agents for paper. The coarse material is used for other purposes, or is reground to yield further fine material.
Relatively large quantities are required for the above-mentioned purposes at the lowest possible price. With existing classifiers of this kind, however, comprising only one fine-material/sifting-air outlet, the throughput is relatively low, being dependent upon, among other things, the dimensions of the classifier, more particularly the length and diameter of the sifting rotor, i.e. the peripheral surface thereof where classifying takes place. Diameter is limited by increasing centrifugal force, while the limiting grain size is dependent upon diameter and r.p.m., and thus also upon centrifugal force. The length of the sifting rotor is also limited, mainly because of the difference in radial flow-velocity along the sifting rotor between pairs of laminations. This radial flow-velocity is dependent upon the suction, i.e. negative pressure applied at the outlet, the said suction being at its maximum at the outlet and decreasing towards the opposite end.